The present invention relates to a bonded fibrous sheet material which is useful, for example, in the manufacture of casings (e.g. skins) for food products.
It is well known that number of food products (particularly certain meat products) are, during their process of manufacture, enclosed within a skin which retains the form or shape of the product. Examples of such food products are sausages, salami etc.
One method which has been used extensively for producing such casings involves viscose treatment of a porous paper web, as described more fully in U.S. Pat. No. 3,135,613. Briefly this process involves the steps of
(a) forming a bonded fibrous web by treatment of the paper with a dilute viscous solution (to apply approximately 1-3% of viscose based on the weight of the paper), drying the web, regenerating cellulose by acid treatment, washing and further drying. The product of this step is a porous, bonded fibrous web having sufficient caustic resistance to withstand the highly alkaline conditions of step (b). Conventionally step (a) has been carried out by the manufacturer of the paper; and
(b) treating the bonded web obtained from step (a) with a highly caustic viscose solution (to apply 300%-400% of viscose by weight of the paper), followed by regeneration of cellulose and washing and drying steps to produce the food casing material.
The product of step (a) has properties rendering it highly desirable for use as a food casing. More particularly, the casings are:
1. porous and permeable to moisture vapor and smoke thus allowing food products enclosed therein to be processed;
2. dimensionally stable to allow food products which may for example be salami of substantial lengths and relatively heavy to be hung without losing their shape; and
3. clear to the extent that the fibrous base cannot be seen.
Such casings are perfectly satisfactory and have been used for many years. However step (1) does have processing disadvantages in that it is a multi-stage process involving dope addition, acidification, neutralisation and washing stages. The multi-stage process associated with step (3) is an accepted process requirement in the industry and is not seen as a particular disadvantage.
Various patents have discussed the use of alternative materials for bonding paper webs to provide an appropriate substrate for casing forming operations where treatment with concentrated viscose solution under highly alkaline conditions is carried out. After undergoing bonding the substrate must retain its porous, absorbent characteristics in order to permit adequate impregnation and encasement by the converted viscose. The bonding agent should also be one that will not cause the substrate to become discoloured during exposure to the conditions of the casing forming process.
U.S. Pat. No. 3,484,256 (Chiu et al) suggests cationic thermosetting resin and polyacrylamide as a replacement for the dilute viscose bonding treatment. U.S. Pat. No. 3,640,734-5 (Conway), and U.S. Pat. No. 3,679,437 (Oppenheimer et al) teach the use of soluble poly (vinyl alcohol) as a wet strengthening agent.
The aforementioned binder materials, whether used alone or in combination frequently provide some but not all of the desired characteristics of the casing. For example the use of poly (vinyl alcohol) having a degree of hydrolysis of about 85% will provide low to moderate dry tensile strengths but poor wet tensile, caustic strength and absorption characteristics. Conversely, the use of fibrous film forming materials such as hydroxyethyl cellulose in conjunction with appropriate cross linking agent such as dialdehyde cross linkers will have the opposite effect from that achieved by the poly(vinyl alcohol). They exhibit good wet tensile strengths and absorbency characteristics but poor caustic tensile strength. Unfortunately, mixtures of these materials also fail to provide all the desired characteristics.
JP-A-6294094 (Oji Paper Co) discloses manufacture of a paper which is stated to have good wet strength and good alkali resistance and which is suitable for use as a casing for a meat product (e.g. ham or sausage) or in the manufacture of tea bags. The paper is produced by adding guar gum and a polyamide epichlorohydrin resin (a wet strength agent) to the wet end of the paper making process. We have however found that papers using this technique do not actually have sufficient wet tensile strength and caustic tensile strength for consistent conversion to food casing material in step (b) outlined above.
WO-A-9510190 (J. R. Crompton Limited) discloses a bonding fibrous sheet material suitable for conversion, by viscose treatment, into a food casing material. The bonded fibrous sheet material is produced by treatment of a porous fibrous substrate (particularly a paper) with a coating composition which under the conditions of the treatment does not form a film and which is an admixture of a polymer latex and a wet strength resin, and effecting cross-linking of the polymer and resin to produce the porous bonded, fibrous sheet material. It is contemplated in WO-A-9510190 that the coating composition may include a fibre consolidation aid, e.g. in an amount of less than 3% by weight of the coating composition. The preferred fibre consolidation aid is carboxymethyl cellulose and other examples given include galactomannan, e.g. guar gum and locust bean gum. We have however found that this binder system can still cause fracture lines and poor body penetration if the latex component is over or under cured. Furthermore, preferred impregnants in accordance with WO-A-9510190 include a surfactant (in addition to the latex and wet-strength resin) and if the surfactant level is not correctly controlled in the impregnant then there is an adverse impact on fracture line propagation.